Certain research and manufacturing processes require the use of a process chamber with high vacuum. The vacuum may be required for any of a number of reasons. In some instances, atmospheric components that could cause a chemical reaction or physical damage during a process must be removed (e.g., in vacuum melting of reactive metals such as titanium). In other instances, vacuum is used to disturb an equilibrium condition existing at normal room conditions, such as in removing volatile liquid or occluded or dissolved gas from the bulk of material (e.g., degassing oils, freeze-drying) or in desorbing gas from surfaces (e.g., the cleanup of microwave tubes during manufacture). Vacuum is also used in processes where the distance must be extended that a particle will travel before it collides with another, thereby permitting the particles to follow a collision-free course between source and target (e.g., in vacuum coating, particle accelerators, television picture tubes). Finally, vacuum is used in preparing clean surfaces, by reducing the number of molecular impacts per second. That decreases the chances of contamination (e.g., in clean-surface studies and preparation of pure, thin films).
In semiconductor wafer processing, vacuum is used during thin-film chemical vapor deposition (CVD) and etching operations, primarily to reduce contamination and to maximize particle trajectories. The vacuum system of the invention, while described herein primarily in connection with a semiconductor wafer manufacturing operation, may be used in other processes and in research activities requiring any of the above uses of vacuum.
Plasma-based semiconductor manufacturing equipment such as a dry etcher typically operates in the 2-250 mTorr pressure range with a carefully controlled vacuum. As shown in the exemplary system 100 of FIG. 1, a process chamber 110 is evacuated by a turbomolecular pump 130. Pressure in the process chamber 110 is typically measured using one or more capacitance manometer instruments 112. The chamber pressure is controlled by feeding manometer measurements into a tool controller 115 that controls and sequences the overall process. The tool controller 115 commands a valve controller 125 that drives a stepper motor on a vane or pendulum-type throttle valve 120 placed between the chamber 110 and the turbomolecular pump 130. Changing the size of the opening of the throttle valve 120 changes the pressure in the chamber 110.
A gas flow rate is set by setting a mass flow control 105 to maintain a constant flow rate through the chamber. The mass flow control maintains a constant mass flow rate as the chamber pressure is adjusted using the throttle valve 120. As used herein, the term “pumping speed” refers to a volumetric gas flow through the pump. That gas flow may be measured, for example, in standard cubic centimeters per minute (SCCM).
Recently, semiconductor processing requirements have forced tool manufacturers to install larger turbomolecular pumps to achieve lower pressures with high gas flows. The required gas flows may sometimes be as high as 1000 SCCM. Additionally, a requirement has arisen for high pressure, low flow conditions in the processing chamber, for process steps such as a breakthrough etch, in-situ photoresist strip or chamber clean.
Throttle valve technology is usually based on a stepper motor actuator having a fixed number of positions that it can assume. For example, a pendulum gate valve is commonly used as a throttle valve in the art. The pendulum gate valve has a pendulum plate that is rotated into and out of sealing position by a servo motor having a finite angular position resolution.
In order to maintain stable pressure control at lower pressures, the valve may be operated in the 20-80% open range. A graphical view of the flow characteristics of a theoretical gate valve is shown in FIG. 2, where a plot 230 represents flow rate 210 as a function valve opening percentage 220. It can be seen that in the 20-80% open range 240, the plot 230 does not have an excessively small or large slope, permitting stable control.
The example system shown is optimized for low pressure, high flow steps in a semiconductor processing recipe where the throttle valve operates close to open to maximize conductance but not so open as to limit control stability.
At higher pressure, high flow conditions the throttle valve is almost completely closed to limit the conductance of gas to the turbomolecular pump. In that region the resolution of pressure control becomes limited, one step of the stepper motor results in a big shift in the conductance and control stability is thereby compromised.
For example, as shown in FIG. 2, to maintain higher pressures in the chamber, the valve must be operated in the 0-20% open range 260. In that region, however, a very small change in valve opening yields a very large change in flow rate, causing instability in the pressure control. That limits the ability to control at higher pressures and lower flows when the valve is throttling a large turbomolecular pump with a large diameter inlet port.
The use of low pressure, high flow and high pressure, low flow steps in the same recipe also presents a significant stability problem for hardware engineers.
The problem has been addressed in several ways, with only limited success. For example, inert gas has been added to the process gas flow to add gas load to the pump. That technique works relatively quickly, and does indeed increase the process chamber pressure without requiring the operation of the throttle valve outside its preferred operating range. The technique, however, significantly increases gas consumption and cost of ownership and may produce unwanted process effects.
The rotational speed of the turbo pump rotor has been varied to adjust process chamber pressure. That technique also produces the desired change in effective pumping speed, but requires significant time to decelerate and accelerate the rotor between high pressure and low pressure steps in the processing recipe. The rotational speed change often takes more than 60 seconds, and presents an unacceptable penalty to productivity.
The backing pump rotational speed has been varied to change process chamber pressure. A backing pump is commonly used downstream of a primary turbomolecular pump to reduce the exhaust pressure of the primary pump. Changing the rotational speed of the backing pump changes the pressure drop across the primary pump, changing the process chamber pressure. That technique also requires excessive time to change the process chamber pressure. Additionally, it requires that the backing pump be located at an exact and consistent distance from the turbo pump for chamber-to-chamber repeatability.
Another approach is to use a more expensive throttle valve having a servo motor with more steps. That solution, however, is undesirable due to the significant cost penalty.
The use of exhaust port throttle valves and other forms of backing pressure control to control the inlet pressure of a turbo pump has been attempted. Each of those cases has focused on the replacement of the inlet throttling gate valve with some arrangement for regulating backing pressure.
There is therefore presently a need to provide a vacuum exhaust apparatus and method that may be used in the higher pressure, lower flow processes that are appearing in the manufacture of semiconductor wafers. Particularly, the technique should be more responsive, and implemented with a lower cost and lower maintenance requirements than those techniques currently used. To the inventors' knowledge, there is currently no such technique available.